Von Bezold Assimilation Effect Reverses in Stereoscopic Conditions

Lightness contrast and lightness assimilation are opposite phenomenon: in contrast, grey targets appear darker when bordering bright surfaces (inducers) rather than dark ones; in assimilation the opposite occurs. The question is: which visual process favours the occurrence of one phenomenon over the other? According to the level of the visual process they refer to, researchers provided three answers to this question. The first asserts that both the phenomena are caused by peripheral processes, the second attributes their occurrence to central processes, and the third claims that contrast involves central processes, whilst assimilation involves peripheral ones.The present research was aimed at testing these hypotheses. An experiment on an IT system equipped with goggles for stereo vision was run. Observers were asked to evaluate the lightness of a grey target, and two variables were systematically manipulated: (i) Inducers’ apparent distance; and (ii) Inducers’ intensity. In all the conditions, the retinal stimulation was kept constant, thus, the peripheral processes remained the same along the experiment. Results show that the lightness of the target depends on both the experimental variables. As the retinal stimulation was kept constant, we conclude that central mechanisms are involved in both the phenomena